1. Field of Invention
The present invention is generally directed to the detection of cardiac function in a patient, and more particularly to the detection of cardiac function and the treatment of cardiac conditions in an ambulatory medical device, such as a wearable defibrillator.
2. Discussion of Related Art
With a portable medical device, such as a wearable defibrillator worn by an ambulatory patient, the patient's electrocardiogram (ECG) signal is obtained from body surface electrodes. When the ECG signal is obtained in this manner, electrical noise or electrode fall-off frequently degrades the quality of the ECG signal. The challenge becomes one of extracting a clean ECG signal from the sometimes noisy signals derived from the body-surface electrodes.
Electrode noise can be caused by electrodes sliding on the patient's body due to extreme patient movement, such as vigorous exercise. Noise can also be caused by a poorly fit electrode belt or garment allowing the electrodes to slide on the patient's body with minor patient movement. Electrode fall-off can be caused by the electrodes flipping over and losing contact with the body, or lifting from the body and losing contact. Even where the electrodes are properly positioned on the patient's body, excessively dry skin can also cause noise.
Known ambulatory wearable defibrillators, such as the LifeVest® Wearable Cardioverter Defibrillator available from Zoll Medical Corporation of Chelmsford, Mass., use four ECG sensing electrodes in a dual-channel configuration. That is, an electrical signal provided by one of the four ECG sensing electrodes is paired with the electrical signal provided by another of the four ECG sensing electrodes to form a channel. This arrangement of ECG sensing electrodes is usually suitable because in most cases it is rare that noise or electrode movement affects the entire body circumference. The dual-channel configuration provides redundancy and allows the system to operate on a single channel if necessary, when one of the channels is declared unusable due to ECG sensing electrode fall-off, or to an inferior signal-to-noise ratio. Because signal quality also varies from patient to patient, having two channels provides the opportunity to have improved signal pickup, since the ECG sensing electrodes are located in different body positions. The two channel system also allows analysis of the ECG signal to determine cardiac conditions as described in U.S. Pat. No. 5,944,669.
A problem with existing electrode systems used in ambulatory medical treatment devices, such as a wearable defibrillator, is that there are still instances where there is noise on both channels. When there is noise or fall-off, the device issues alarms so that the patient can take action to correct the problem. With a noisy ECG signal, the arrhythmia detection algorithm in the wearable defibrillator device can be “fooled” into detecting the noise as an arrhythmia, thereby causing the device to issue a treatment sequence that, if not terminated by the patient, could deliver an unnecessary shock.